Field of Invention
The present invention relates tunneling shield structure, and more particularly to a hybrid-type tunnel boring apparatus.
Description of Related Arts
A conventional tunnel boring apparatus is for excavating a tunnel. A hybrid-type tunnel boring apparatus is particularly suitable for excavating tunnel through unstable and mixed terrain consisting of clay, sandy soil, weathered rocks, and sandy gravel stratum. A conventional hybrid-type tunnel boring apparatus usually comprises a shield and a rotating cutting wheel. The shield usually has a front shield portion, a mid shield portion and a rear shield portion. The rotating cutting wheel is usually positioned at the forefront of the tunnel boring apparatus and arranged to be in contact with the stratum for excavating thereof. The rotating cutting wheel of the hybrid-type tunnel boring apparatus is usually equipped with a wide variety of cutting tools for excavating mixed ground. Thus, the rotating cutting wheel may comprise cutting bits and scrapers designed for soft ground tunneling, and disc cutters designed for hard ground tunneling. For this type of rotating wheel, when it is used to excavate soft ground, the disc cutter may have a friction which is less than a starting resistance of the disc cutter. When this is the case, the disc cutter may not be started properly. Moreover, when the disc cutter is used on soft terrain with liquid, the excavated material may reside on the disc cutter which may severely affect excavating performance or permanently damage the cutter head. This phenomenon increases the frequency of which the cutter head must be replaced, and prolongs the completion time of the overall excavation process.